1. Field of the Invention
The invention relates to shock absorbing subassemblies used in oil and gas well drilling operations, which are used for absorbing and reducing vibrations, shocks, or impact loads imparted to the drilling string from the drill bit during drilling operations.
2. Description of the Prior Art
In drilling oil and gas wells on land or in offshore environments, the drill bit generates considerable impact loads, shocks, and vibrations which, if allowed to be transmitted into the drilling string through the drill collar connecting the string and bit, could cause metal fatigue and eventual cracking and failure of the collar. Additionally, the impact loads, shocks, and/or vibrations, affect the length of the drill bit life, and prevent the drill bit from being continuously maintained against the bottom of the hole being drilled.
Accordingly, the art has provided shock absorbing subassemblies disposed in the drilling string above the drilling bit to isolate bit induced vibration, shocks, and impact loads from the drill string above the subassembly. Such shock absorbing subassemblies utilize a splined engagement between a mandrel and an elongated body, whereby drilling torque is transmitted through the splined engagement between the mandrel and the body. The splined section also permits upward and downward movement of the body, with respect to the mandrel, which in turn applies impact loads to the shock absorber element of the shock absorbing subassembly. An illustrative example of such a shock absorbing subassembly is that disclosed in U.S. Pat. No. 3,406,537, issued to C. B. Falkner, Jr. on Oct. 22, 1968. The shock absorbing subassembly disclosed therein, utilizes a plurality of discs made of stainless steel wire, woven into a cloth-like mesh. Other shock absorbing subassemblies known in the art utilized helical springs, blocks of elastomer materials, or thin elastomer rings covered by separate, special shaped steel cups, as the shock absorber elements.
One of the problems associated with shock absorbing subassemblies, which utilize elastomer materials or woven wire mesh material as the shock absorber element, is that after a period of time such materials may take a permanent set, whereby the spring constants of these materials may greatly increase. Accordingly, a much greater load would need to be applied to the shock absorber in order to give the desired compression of the shock absorber element and deflection between the mandrel and body. In other words, the taking of a permanent set of the shock absorber element results in reduced elasticity thereof, which results in poorer shock absorbing characteristics of the shock absorber subassembly.
In the case of shock absorbing subassemblies which utilize helical springs as the shock absorber element, the use of such springs having high spring constants provides adequate dampening characteristics for high impact loads; however, the deflection and dampening characteristics for lower loads may be unsatisfactory. If a helical spring having a low spring constant is utilized as the shock absorber element, adequate dampening and deflection characteristics are exhibited in the lower impact load range; however, in the high impact load range a much longer spring must be utilized to provide the necessary dampening affect. The use of such an elongated spring necessitates a greatly increased length for the body and mandrel, whereby the costs of the shock absorbing subassembly are increased.
Accordingly, prior to the development of the present invention, there has been no shock absorbing subassembly available which utilizes a shock absorber element which does not take a permanent set, has the shortest possible subassembly length for greater economy and ease of handling and serviceability, and provides efficient absorption of vibrations, shocks, and impact loading, over high and low ranges of impact loading. Therefore, the art has sought an efficient, durable, economical, short, easily handled, easily serviceable, and versatile shock absorbing subassembly for use in oil and gas well drilling operations.